The Collaborative International Dictionary of English v.0.48:

Wacke \Wack"e\, Wacky \Wack"y\, n. [G. wacke, MHG. wacke a large
   stone, OHG. waggo a pebble.] (Geol.)
   A soft, earthy, dark-colored rock or clay derived from the
   alteration of basalt.
   [1913 Webster]